Injection Device

ABSTRACT

An injection device ( 210; 110 ) is described. A housing ( 212; 112 ) receives a syringe and includes a return spring ( 226; 126 ) for biasing the syringe from an extended position in which its needle ( 218; 118 ) extends from the housing ( 212; 112 ) to a retracted position in which the it does not. A drive spring ( 230; 130 ) acts on a first drive element ( 232; 132 ) and a second drive element ( 234; 134 ) acts upon the syringe to advance it from its retracted position to its extended position and discharge its contents through the needle. The first drive element ( 232; 132 ) is capable of movement relative to the second ( 234; 134 ) once a nominal decoupling position has been reached. A release mechanism is activated when the first drive element ( 234; 134 ) is further advanced to a nominal release position, to release the syringe ( 214; 114 ) from the action of the drive spring ( 230; 130 ), whereupon the return spring ( 226; 126 ) restores the syringe ( 214; 114 ) to its retracted position. The nominal decoupling and release positions are defined relative to the syringe ( 214; 114 ). This may be achieved by interaction between a moving component and a decoupling component ( 162; 262 ) that moves with the syringe as it is advanced.

BACKGROUND TECHNOLOGY

The present invention relates to an injection device of the type thatreceives a syringe, extends it, discharges its contents and thenretracts it automatically. Devices of this general description are shownin WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring andsome form of release mechanism that releases the syringe from theinfluence of the drive spring once its contents are supposed to havebeen discharged, to allow it to be retracted by a return spring.

Because of the stack-up of tolerances of the various components of thedevice, a certain margin of safety must be built into the activation ofthe release mechanism, to ensure that it is effective. The consequenceof underestimating the safety margin is that the release mechanism mayfail to operate even once the syringe contents have been discharged,which is unsatisfactory in a device that is supposed to retractautomatically, particularly for self-administered drugs. On the otherhand, overestimating the safety margin may mean that some of the syringecontents are discharged after the syringe has retracted, which resultsfirstly in a short dose and secondly in what may be termed a “wet”injection. Wet injections are undesirable for the squeamish,particularly in connection with self-administered drugs.

UK patent applications nos. 0210123, 0229384 and 0325596 describe aseries of injection devices designed to deal with this problem. Eachmakes use of a neat trick that delays the release of the syringe for acertain period of time after the release mechanism has been activated,in an attempt to ensure that the syringe has been completely discharged.The devices illustrated in UK patent application no. 0325596 make use ofa two-part drive incorporating a fluid-damped delay mechanism that isparticularly effective in ensuring complete discharge of the syringecontents. In each case, the device relies upon two unlatchingmechanisms. The first unlatching mechanism initiates the fluid dampingmechanism and the second releases the syringe from the actuator,allowing it to be withdrawn. The unlatching mechanisms are activated bycomponents of the injection device having been advanced to nominalunlatching positions relative to the device casework.

A device 10 of this general character is illustrated schematically inFIG. 1. The sequence of operation is as follows. Firstly, the device 10is armed. The user presses a release button and the syringe 14 isadvanced a distance d₁ by a drive spring 30, thereby compressing theretraction spring 26. This movement inserts the needle 18 into thepatient. The plunger 23 is advanced a distance d₂ by the drive spring30, injecting most of the dose. Once nearly the entire dose has beeninjected, the first unlatching mechanism is activated, an operationillustrated schematically by the coincidence of components 1 and 3. Theplunger 23 is then advanced a further distance d₃ by the drive spring30, injecting the rest of the dose. Finally, the second unlatchingmechanism is activated, an operation illustrated schematically by thecoincidence of components 2 and 4, and the retraction spring 26 thencauses the needle 18 to be retracted by the distance d₁.

Since the drive spring acts upon the same component of the devicethroughout, here referred to as the “actuator”, the distance thatcomponent must move between the device being armed and the secondunlatching mechanism being activated is, subject to tolerance stack-up,equal to the sum of d₁, d₂ and d₃. In the devices described in theapplications mentioned above, all of this movement takes place to therear of the syringe, which means that the overall length of the devicemust be greater than the sum of the length of the actuator, thedistances d₁, d₂ and d₃ and the length of the syringe body not includingthe needle.

The best design of injection device is one that is compact. This isimportant both to the ergonomics of the device and to its manufacturedcost. The length of the device can be reduced by allowing the actuatorto move past the syringe, and by having the unlatching mechanismsactivated in front of the syringe. However, this would require theactuator and its unlatching mechanisms to pass around the space occupiedby the syringe, involving an increase in diameter of the device thatnegates the length savings.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a more compactdevice. Instead of triggering release of the unlatching mechanisms usinga fixed point on the device casework, the present invention does itusing one of more features that move forward with the syringe as it isadvanced. In other words, the nominal positions at which the unlatchingmechanisms are activated are defined relative to the syringe, notrelative to the device casework. As illustrated in FIG. 2, these nominalpositions also move forwards a distance d₁ as the syringe is initiallyadvanced. This in turn means that the initial distance between theactuator and the syringe plunger can be reduced by the distance d₁. Thelength of the device can be reduced by d₁ at a stroke. More modestimprovements are available when only one of the nominal positions atwhich the unlatching mechanisms are activated is defined relative to thesyringe.

Accordingly, a first aspect of the present invention provides aninjection device comprising:

-   -   a housing adapted to receive a syringe having a discharge        nozzle;    -   first and second drive elements, of which the first is acted        upon and the second acts upon the syringe to advance it from its        retracted position to its extended position and discharge its        contents through the discharge nozzle, the first drive element        being capable of movement relative to the second when the former        is acted upon and the latter is restrained by the syringe;    -   a coupling that prevents the first drive element from moving        relative to the second until they have been advanced to a        nominal decoupling position relative to the syringe.

In this case, the nominal decoupling position, i.e. the first nominalunlatching position, is defined relative to the syringe and not relativeto the housing.

Preferably, the device includes:

-   -   an actuator that acts upon the first drive element;    -   means for biasing the syringe from an extended position in which        the discharge nozzle extends from the housing to a retracted        position in which the discharge nozzle is contained within the        housing; and    -   a release mechanism, activated when the first drive element has        been advanced to a nominal release position that is more        advanced than the said nominal decoupling position, and adapted        to release the syringe from the action of the actuator,        whereupon the biasing means restores the syringe to its        retracted position.

In preferred embodiments of the invention, the nominal decouplingposition is defined either by one of the drive elements interacting witha decoupling component that moves with the syringe as it is advanced.

For ease of manufacture and assembly, the coupling may comprise flexiblearms on one of the drive elements that engage with a drive surface onthe other, in which case the decoupling component causes the flexiblearms to move when the said nominal decoupling position is reached, thusdisengaging them from the drive surface to allow the first drive elementto move relative to the second.

A second aspect of the present invention provides an injection devicecomprising:

-   -   a housing adapted to receive a syringe having a discharge        nozzle, the housing including means for biasing the syringe from        an extended position in which the discharge nozzle extends from        the housing to a retracted position in which the discharge        nozzle is contained within the housing;    -   first and second drive elements, of which the first is acted        upon and the second acts upon the syringe to advance it from its        retracted position to its extended position and discharge its        contents through the discharge nozzle, the first drive element        being capable of movement relative to the second when the former        is acted upon and the latter is restrained by the syringe;    -   a coupling that prevents the first drive element from moving        relative to the second until they have been advanced to a        nominal decoupling position; and    -   a release mechanism, activated when the first drive element has        been advanced to a nominal release position relative to the        syringe that is more advanced than the said nominal decoupling        position, and adapted to release the syringe, whereupon the        biasing means restores the syringe to its retracted position.

Here, the nominal release position, i.e. the second nominal unlatchingposition, is defined relative to the syringe and not relative to thehousing.

Again, in preferred embodiments, the nominal release position is definedby an actuator or the first drive element interacting with a decouplingcomponent that moves with the syringe as it is advanced. It may bedefined by the actuator interacting with the first drive element oncethe nominal decoupling position has been reached, at which position thefirst drive element is restrained by the syringe against furthermovement.

Once again, for ease of manufacture and assembly, of the actuator andthe first drive element, one preferably comprises second flexible armsthat engage with a second drive surface on the other, and the releasemechanism preferably comprises the said decoupling component, whichcauses the second flexible arms to move when the said nominal releaseposition is reached, thus disengaging them from the drive surface.

Alternatively, of an actuator and the first drive element, onepreferably comprises second flexible arms that engage with a seconddrive surface on the other, allowing the actuator to act upon the firstdrive element and preventing the former from moving relative to thelatter until the nominal release position has been reached, the secondflexible arms are preferably biased toward a position at which theyengage the second drive surface and the release mechanism preferablycauses them to move against their bias, thus disengaging them from thedrive surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIGS. 1 and 2 are schematic illustrations to which reference has alreadybeen made;

FIG. 3 is an illustration of a first embodiment of the invention; and

FIG. 4 is likewise a second.

DETAILED DESCRIPTION

FIG. 3 shows an injection device 110 in which a housing 112 contains ahypodermic syringe 114. The syringe 114 is of conventional type,including a syringe body 116 terminating at one end in a hypodermicneedle 118 and at the other in a flange 120. The conventional plungerthat would normally be used to discharge the contents of the syringe 114manually has been removed and replaced with a drive element 134 as willbe described below, to which is attached a bung 122. The bung 122constrains a drug 124 to be administered within the syringe body 116.Whilst the syringe illustrated is of hypodermic type, this need notnecessarily be so. Transcutaneous or ballistic dermal and subcutaneoussyringes may also be used with the injection device of the presentinvention. Generally, the syringe must include a discharge nozzle, whichin a hypodermic syringe is the needle 118.

As illustrated, the housing includes a return spring 126 that biases thesyringe 114 from an extended position in which the needle 118 extendsfrom an aperture 128 in the housing 112 to a retracted position in whichthe discharge nozzle 118 is contained within the housing 112. The returnspring 126 acts on the syringe 114 via a sleeve 127.

At the other end of the housing is a compression drive spring 130. Drivefrom the drive spring 130 is transmitted via a multi-component drive tothe syringe 114 to advance it from its retracted position to itsextended position and discharge its contents through the needle 118. Thedrive accomplishes this task by acting directly on the drug 124 and thesyringe 114. Hydrostatic forces acting through the drug 124 and, to alesser extent, static friction between the bung 122 and the syringe body116 initially ensure that they advance together, until the return spring126 bottoms out or the syringe body 116 meets some other obstructionthat retards its motion.

The multi-component drive between the drive spring 130 and the syringe114 consists of three principal components. A drive sleeve 131 takesdrive from the drive spring 130 and transmits it to flexible latch arms133 on a first drive element 132. This in turn transmits drive viaflexible latch arms 135 to a second drive element, the drive element 134already mentioned.

The first drive element 132 includes a hollow stem 140, the inner cavityof which forms a collection chamber 142 in communication with a vent 144that extends from the collection chamber through the end of the stem140. The second drive element 134 includes a blind bore 146 that is openat one end to receive the stem 140 and closed at the other. As can beseen, the bore 146 and the stem 140 define a fluid reservoir 148, withinwhich a damping fluid is contained.

A trigger (not shown) is provided on one side of the housing 112. Thetrigger, when operated, serves to decouple the drive sleeve 131 from thehousing 112, allowing it to move relative to the housing 112 under theinfluence of the drive spring 130. The operation of the device is thenas follows.

Initially, the drive spring 130 moves the drive sleeve 131, the drivesleeve 131 moves the first drive element 132 and the first drive element132 moves the second drive element 134, in each case by acting throughthe flexible latch arms 133, 135. The second drive element 134 and thebung 122 move and, by virtue of static friction and hydrostatic forcesacting through the drug 124 to be administered, move the syringe body116 against the action of the return spring 126. The return spring 126compresses and the hypodermic needle 118 emerges from the exit aperture128 of the housing 112. This continues until the return spring 126bottoms out or the syringe body 116 meets some other obstruction thatretards its motion. Because the static friction between the bung 122 andthe syringe body 116 and the hydrostatic forces acting through the drug124 to be administered are not sufficient to resist the full drive forcedeveloped by the drive spring 130, at this point the second driveelement 134 begins to move within the syringe body 116 and the drug 124begins to be discharged. Dynamic friction between the bung 122 and thesyringe body 116 and hydrostatic forces acting through the drug 124 tobe administered are, however, sufficient to retain the return spring 126in its compressed state, so the hypodermic needle 118 remains extended.

Before the second drive element 134 reaches the end of its travel withinthe syringe body 116, so before the contents of the syringe have fullydischarged, the flexible latch arms 135 linking the first and seconddrive elements 132, 134 reach a constriction 137. The constriction 137is formed by a component 162 that is attached to the syringe flange 120,so it will be understood that when the syringe 114 advances from itsretracted position to its extended position, the component 162 advanceswith it. The constriction 137 moves the flexible latch arms 135 inwardsfrom the position shown to a position at which they no longer couple thefirst drive element 136 to the second drive element 134, aided by thebevelled surfaces on the constriction 137.

Once this happens, the first drive element 136 acts no longer on thesecond drive element 134, allowing the first drive element 132 to moverelative to the second drive element 134.

Because the damping fluid is contained within a reservoir 148 definedbetween the end of the first drive element 132 and the blind bore 146 inthe second drive element 134, the volume of the reservoir 148 will tendto decrease as the first drive element 132 moves relative to the seconddrive element 134 when the former is acted upon by the drive spring 130.As the reservoir 148 collapses, damping fluid is forced through the vent144 into the collection chamber 142. Thus, once the flexible latch arms135 have been released, the force exerted by the drive spring 130 doeswork on the damping fluid, causing it to flow through the constrictionformed by the vent 144, and also acts hydrostatically through the fluid,to drive the second drive element 134. Losses associated with the flowof the damping fluid do not attenuate the force acting on the body ofthe syringe to a great extent. Thus, the return spring 126 remainscompressed and the hypodermic needle 118 remains extended.

After a time, the second drive element 134 completes its travel withinthe syringe body 116 and can go no further. At this point, the contentsof the syringe 114 are completely discharged and the force exerted bythe drive spring 130 acts to retain the second drive element 134 in itsterminal position and to continue to cause the damping fluid to flowthrough the vent 144, allowing the first drive element 132 to continueits movement.

Before the reservoir 148 of fluid is exhausted, the flexible latch arms133 linking the drive sleeve 131 with the first drive element 132 reachanother constriction 139, also provided by the component 162 that isattached to the syringe flange 120. The constriction 139 moves theflexible latch arms 133 inwards from the position shown to a position atwhich they no longer couple the drive sleeve 131 to the first driveelement 132, aided by the bevelled surfaces on the constriction 139.Once this happens, the drive sleeve 131 acts no longer on the firstdrive element 132, allowing them to move relative to each other. At thispoint, of course, the syringe 114 is released, because the forcedeveloped by the drive spring 130 is no longer being transmitted to thesyringe 114, and the only force acting on the syringe will be the returnforce from the return spring 126. Thus, the syringe 114 now returns toits retracted position and the injection cycle is complete.

All this takes place, of course, only once the cap 111 has been removedfrom the end of the housing 112. As can be seen from FIG. 3, the end ofthe syringe is sealed with a boot 123. The central boss 121 of the cap111 is hollow at the end and a lip 125 of the hollow end is bevelled onits leading edge 157, but not its trailing edge. Thus, as the cap 111 isinstalled, the leading edge 157 of the lip 125 rides over a shoulder 159on the boot 123. However, as the cap 111 is removed, the trailing edgeof the lip 125 will not ride over the shoulder 159, which means that theboot 123 is pulled off the syringe 114 as the cap 111 is removed.

FIG. 4 shows another injection device 210 in which a housing 212contains a hypodermic syringe 214. The syringe 214 is again ofconventional type, including a syringe body 216 terminating at one endin a hypodermic needle 218 and at the other in a flange 220, and arubber bung 222 that constraints a drug 224 to be administered withinthe syringe body 216. The conventional plunger that would normally beconnected to the bung 222 and used to discharge the contents of thesyringe 214 manually, has been removed and replaced with amulti-component drive element as will be described below. Whilst thesyringe illustrated is again of hypodermic type, this need notnecessarily be so. As illustrated, the housing includes a return spring226 that biases the syringe 214 from an extended position in which theneedle 218 extends from aperture 228 in the housing 212, to a retractedposition in which the hypodermic needle 218 is contained within thehousing 212. The return spring 226 acts on the syringe 214 via a sleeve227.

At the other end of the housing is a compression drive spring 230. Drivefrom the drive spring 230 this transmitted via the multi-component driveto the syringe 214 to advance it from its retracted position to itsextended position and discharge its contents through the needle 218. Thedrive accomplishes this task by acting directly on the drug 224 and thesyringe 214. Static friction between the bung 222 and the syringe body216 initially ensures that they advance together, until the returnspring 226 bottoms out or the syringe body 216 meets some otherobstruction that retards its motion.

The multi component drive between the drive spring 230 and the syringe214 again consists of three principal components. The drive sleeve 231takes drive from the drive spring 230 and transmits it to flexible latcharms 233 on a first drive element 232. These elements are shown indetail “A”. The first drive element 232 in turn transmits drive viaflexible latch arms 235 to a second drive element 234. These elementsare shown in detail “B”. As before, the first drive element 232 includesa hollow stem 240, the inner cavity of which forms a collection chamber242. The second drive element 234 includes a blind for 246 that is openat one end to receive the stem 240 and closed at the other. As can beseen, the bore 246 and the stem 240 define a fluid reservoir 248, withinwhich a damping fluid is contained.

A trigger (not shown) is provided in the middle of the housing 212. Thetrigger, one operated, serves to decouple the drive sleeve 231 from thehousing 212 allowing it to move relative to the housing 212 under theinfluence of the drive spring 230. The operation of the device is thenas follows.

Initially, the drive spring 230 moves the drive sleeve 231, the drivesleeve 231 moves the first drive element 232 and the first drive element232 moves the second drive element 234, in each case by acting throughthe flexible matching arms 233, 235. The second drive element 234 movesand, by virtue of static friction and hydrostatic forces acting throughthe drug 224 to be administered, moves the syringe body 216 against theaction of the return spring 226. The return spring 226 compresses andthe hypodermic needle 218 emerges from the exit aperture 228 of thehousing 212. This continues until the return spring 226 bottoms out orthe syringe body 216 meets some other obstruction that retards itsmotion. Because the static friction between the bung 222 and the syringebody 216 and the hydrostatic forces acting through the drug 224 to beadministered are not sufficient to resist the full drive force developedby the drive spring 230, at this point the second drive element 234begins to move within the syringe body 216 and the drug 224 begins to bedischarged. Dynamic friction between the bung 222 and the syringe body216 and hydrostatic forces acting through the drug 224 to beadministered are, however, sufficient to retain the return spring 226 inits compressed state, so the hypodermic needle 218 remains extended.

Before the second drive element 234 reaches the end of its travel withinthe syringe body 216, so before the contents of the syringe have fullydischarged, the flexible latch arms 235 linking the first and seconddrive elements 232, 234 reach a constriction 237. The constriction 237is formed by a component 262 that is attached to the syringe carrier.Additional flexible arms 247 overlie the flexible arms 235 on the firstdrive element 232, by means of which drive is transmitted to the seconddrive element 234. FIG. 4 illustrates the injection device 210 at theposition where the additional flexible arms 247 are just making contactwith the constriction 237 in the component 262.

The constriction 237 moves the additional flexible arms 247 inwards,aided by the bevelled surfaces on both, and the additional flexible arms247 in turn move the flexible arms 235, by means of which drive istransmitted from the first drive element 232 to the second drive element234, inwards from the position shown to a position at which they nolonger couple the first and second drive elements together. Once thishappens, the first drive element 232 acts no longer on the second driveelement 234, allowing the first drive element 232 to move relative tothe second drive element 234.

Because the damping fluid is contained within a reservoir 248 definedbetween the end of the first drive element 232 and the blind bore 246 inthe second drive element 234, the volume of the reservoir 248 will tendto decrease as the first drive element 232 moves relative to the seconddrive element 234 when the former is acted upon by the drive spring 230.As the reservoir 248 collapses, damping fluid is forced into thecollection chamber 242. Thus, once the flexible latch arms 235 have beenreleased, the force exerted by the drive spring 230 does work on thedamping fluid, causing it to flow into the collection chamber 242, andalso acts hydrostatically through the fluid, thence via the second driveelement 234. Losses associated with the flow of the damping fluid do notattenuate the force acting on the body of the syringe to a great extent.Thus, the return spring 226 remains compressed and the hypodermic needleremains extended.

After a time, the second drive element 234 completes its travel withinthe syringe body 216 and can go no further. At this point, the contentsof the syringe 214 are completely discharged and the force exerted bythe drive spring 230 acts to retain the second drive element 234 in itsterminal position and to continue to cause the damping fluid to flowinto the collection chamber 142, allowing the first drive element 232 tocontinue its movement.

A flange 270 on the rear of the second drive element 234 normallyretains the flexible arms 233 in engagement with the drive sleeve 231.However, before the reservoir 248 of fluid is exhausted, the flexiblelatch arms 233 linking the drive sleeve 231 with the first drive element232 move sufficiently far forward relative to the second drive element234 that the flange 270 is brought to register with a rebate 272 in theflexible arms 233, whereupon it ceases to be effective in retaining theflexible arms 233 in engagement with the drive sleeve 231. Now, thedrive sleeve 231 moves the flexible latch arms 233 inwards from theposition shown to a position at which they no longer couple the drivesleeve 231 to the first drive element 232, aided by the bevelledlatching surfaces 274 on the flexible arms 233. Once this happens, thedrive sleeve 231 acts no longer on the first drive element 232, allowingthem to move relative to each other. At this point, of course, thesyringe 214 is released, because the forces developed by the drivespring 230 are no longer being transmitted to the syringe 214, and theonly force acting on the syringe will be the return force from thereturn spring 226. Thus, the syringe 214 now returns to its retractedposition and the injection cycle is complete.

1. An injection device comprising: a housing adapted to receive asyringe having a discharge nozzle; first and second drive elements, ofwhich the first is acted upon and the second acts upon the syringe toadvance it from its retracted position to its extended position anddischarge its contents through the discharge nozzle, the first driveelement being capable of movement relative to the second when the formeris acted upon and the latter is restrained by the syringe; and acoupling that prevents the first drive element from moving relative tothe second until they have been advanced to a nominal decouplingposition relative to the syringe.
 2. An injection device according toclaim 1 including: an actuator that acts upon the first drive element;means for biasing the syringe from an extended position in which thedischarge nozzle extends from the housing to a retracted position inwhich the discharge nozzle is contained within the housing; and arelease mechanism, activated when the first drive element has beenadvanced to a nominal release position that is more advanced than thesaid nominal decoupling position, and adapted to release the syringefrom the action of the actuator, whereupon the biasing means restoresthe syringe to its retracted position.
 3. An injection device accordingto claim 1 in which the nominal decoupling position is defined by one ofthe drive elements interacting with a decoupling component that moveswith the syringe as it is advanced.
 4. An injection device accordingclaim 1 in which: the coupling comprises cooperating features of thefirst and second drive elements that allow the first to act upon thesecond.
 5. An injection device according to claim 4 in which thecooperating features include flexible arms on one of the drive elementsthat engage with a drive surface on the other.
 6. An injection deviceaccording to claim 1 in which the coupling comprises a decouplingmechanism, activated when the drive elements have been advanced to thesaid nominal decoupling position and adapted to decouple the first driveelement from the second, thus allowing the first drive element to moverelative to the second.
 7. An injection device according to claim 3, inwhich: the coupling comprises flexible arms on one of the drive elementsthat engage with a drive surface on the other; and the decouplingcomponent causes the flexible arms to move when the said nominaldecoupling position is reached, thus disengaging them from the drivesurface to allow the first drive element to move relative to the second.8. An injection device according to claim 4, in which: the couplingcomprises flexible arms on one of the drive elements that engage with adrive surface on the other; and the decoupling component causes theflexible arms to move when the said nominal decoupling position isreached, by acting on an intermediate component, thus disengaging theflexible arms from the drive surface to allow the first drive element tomove relative to the second.
 9. An injection device according to claim 8in which the intermediate component is a flexible component of the driveelement upon which the said drive surface is to be found.
 10. Aninjection device according to any one of claim 5 in which the flexiblearms are biased toward a position at which they engage the drive surfaceand the decoupling component causes them to move against their bias,thus disengaging them from the drive surface.
 11. An injection devicecomprising: a housing adapted to receive a syringe having a dischargenozzle, the housing including means for biasing the syringe from anextended position in which the discharge nozzle extends from the housingto a retracted position in which the discharge nozzle is containedwithin the housing; first and second drive elements, of which the firstis acted upon and the second acts upon the syringe to advance it fromits retracted position to its extended position and discharge itscontents through the discharge nozzle, the first drive element beingcapable of movement relative to the second when the former is acted uponand the latter is restrained by the syringe; a coupling that preventsthe first drive element from moving relative to the second until theyhave been advanced to a nominal decoupling position; and a releasemechanism, activated when the first drive element has been advanced to anominal release position relative to the syringe that is more advancedthan the said nominal decoupling position, and adapted to release thesyringe, whereupon the biasing means restores the syringe to itsretracted position.
 12. An injection device according to claim 11, inwhich the release mechanism is activated when the first drive elementhas been advanced to a nominal release position relative to the syringe.13. An injection device according to claim 11 in which the nominalrelease position is defined by the first drive element, or an actuatorthat acts upon it, interacting with a decoupling component that moveswith the syringe as it is advanced.
 14. An injection device according toclaim 11 in which the nominal release position is defined by an actuatorinteracting with the first drive element once the nominal decouplingposition has been reached.
 15. An injection device according to claim 11in which the release mechanism is adapted to decouple the first driveelement from an actuator once the said nominal release position has beenreached, thus releasing the syringe from the action of the actuator. 16.An injection device according claim 11, further comprising a secondcoupling, between an actuator and the first drive element, that preventsthe actuator from moving relative to the first drive element until thenominal release position has been reached.
 17. An injection deviceaccording to claim 16 in which the second coupling comprises cooperatingfeatures of the actuator and the first drive element allowing the formerto act upon the latter.
 18. An injection device according to claim 17 inwhich: the cooperating features of the actuator and the first driveelement include second flexible arms on one of them engaged with asecond drive surface on the other; and the release mechanism comprises adecoupling component that causes the second flexible arms to move whenthe said nominal release position is reached, thus disengaging them fromthe drive surface to allow the actuator to move relative to the firstdrive element.
 19. An injection device according to claim 13, in which:of the actuator and the first drive element, one comprises secondflexible arms that engage with a second drive surface on the other,allowing the actuator to act upon the first drive element and preventingthe former from moving relative to the latter until the nominal releaseposition has been reached; the release mechanism comprises the saiddecoupling component, which causes the second flexible arms to move whenthe said nominal release position is reached, thus disengaging them fromthe drive surface to allow the actuator to move relative to the firstdrive element.
 20. An injection device according to claim 14, in which:of the actuator and the first drive element, one comprises secondflexible arms that engage with a second drive surface on the other,allowing the actuator to act upon the first drive element and preventingthe former from moving relative to the latter until the nominal releaseposition has been reached; and the second flexible arms are biasedtoward a position at which they engage the second drive surface and therelease mechanism causes them to move against their bias, thusdisengaging them from the drive surface.